What type of infectious agent consists of submicroscopic particles that infect cells?

The correct answer is the virus. Viruses are unique infectious agents that are composed of submicroscopic particles that cannot reproduce independently outside of a host cell. They consist of genetic material encased in a protein coat and sometimes surrounded by a lipid envelope. When a virus infects a host cell, it injects its genetic material into the cell, hijacking the cell's machinery to replicate itself.

In contrast, bacteria are single-celled organisms that can live independently and reproduce on their own. Fungi, which include molds, yeasts, and mushrooms, are more complex organisms that can exist in various forms and typically do not require a host to grow. Protozoa are also single-celled and are classified as more complex than viruses and can thrive in a variety of environments, including as parasites in a host. This distinction is crucial because it underscores the unique nature of viruses compared to other infectious agents.